This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Application No. 11(1999)-324707 filed on Nov. 15, 1999, the entire content of which is incorporated herein by reference.
This invention generally relates to electromagnets. More particularly, the present invention pertains to an electromagnet used for a solenoid valve for controlling a hydraulic pressure circuit.
Electromagnet have been used in the past for solenoid valves. One such type of solenoid valve is disclosed in Japanese Patent Laid-Open No. Hei 10-299932. This solenoid valve includes a cylindrically shaped bobbin wound on its outer periphery with an electric coil, a yoke disposed at an axial end in a longitudinal direction of the electric coil and an inner peripheral portion of the bobbin and forming a magnetic circuit in cooperation with a case made of magnetic material, and a plunger disposed in the inner peripheral portion of the yoke and slidably movable in the yoke.
The bobbin is formed by inserting fixed cores and the plunger is held on the inner peripheral faces of the bobbin. The yoke made of magnetic material is disposed at both ends in the longitudinal direction of the cylindrical coil wound around the bobbin made of resin and is disposed in an inner peripheral portion of the bobbin. A space between the inner peripheral portion of the bobbin and the yoke does not exist as the yoke and the bobbin are made as one unit by insert forming.
However, in the above-described solenoid valve, the outer periphery of the yoke facing the inner peripheral portion of the bobbin is smooth, and the thickness of the cylindrical portion of the bobbin is relatively thick in order to ensure the necessary strength for the solenoid valve. Therefore, the magnetic efficiency of the solenoid valve is not capable of being improved beyond a certain level. Also, it is possible that the bobbin may become broken away from the yoke by virtue of the heat and/or vibration that may exist in the surrounding environment in which the electromagnet is used (e.g., in a solenoid valve in a vehicle).
A need thus exists for an improved electromagnet and solenoid valve which overcome the above drawbacks.
A need also exists for a solenoid valve having improved magnetic efficiency and capable of inhibiting or preventing the bobbin from becoming broken away from the yoke by heat impact.
In accordance with an aspect of this invention, an electromagnet includes a cylindrically shaped electric coil wound around a bobbin made of resin material, a cylindrically shaped case made of magnetic material and housing the electric coil, a yoke made of magnetic material for forming a magnetic circuit in cooperation with the case, with both longitudinal ends of the yoke facing the bobbin, and a plunger made of magnetic material and disposed in the inner peripheral of the yoke for moving by electromagnetic force in the longitudinal direction when current is supplied to the electric coil. The bobbin and the yoke are integrated with each other by insert forming, with the outer peripheral portion of the yoke facing the inner peripheral portion of the bobbin being subjected to grooved treatment in the longitudinal direction to form at least one groove and/or knurling treatment to form a knurled outer peripheral portion of the yoke.